THE AGGREGATION OF AQUEOUS DODECYLPHOSPHONIC ACID IN DODECYLTRIMETHYLAMMONIUM HYDROXIDE MIXTURES

The aggregation of aqueous dodecylphosphonic acid (DPA) and do decyl t rimethylammonium hydroxide (DTAOH) mixtures was studied by several met hods. The behavior of DPA-rich mixtures is close to that of pure DPA. This is probably due to the preservation of the hydrogen-bonded struct ure of the micellar headgroup layer. The behavior is almost ideal. Bet ween y(DPA) = 0.5 and 0.33 (y(DPA) being the mole fraction of DPA in t he surfactant mixture), the hydrogen-bonded structure of the micellar headgroup layer is destroyed. A sort of ''micellar azeotrope'' is form ed, and the maximum of non-ideal interaction between the two surfactan ts is attained at y(DPA) = 0.4. For y(DPA) < 0.33 the system behaves a s a common mixture of a cationic surfactant and a non-ionic one (DPA.2 LTA). There is a phenomenon of counterion ''condensation'' on aggregat es at concentrations over the CMC.